Journal of Molecular Spectroscopy最新文献

For three rare isotopologues of carbon dioxide, ¹⁸O¹²C¹⁸O (828, according to a well-established shorthand notation), ¹⁷O¹²C¹⁸O (728), and ¹⁸O¹³C¹⁸O (838), 3923, 4318, and 1058 empirical rovibrational energy levels, respectively, are determined, using the MARVEL (Measured Active Rotational–Vibrational Energy Levels) protocol and code. For the isotopologues 828/728/838, the analysis of their spectroscopic network is based on 11 353(7665)/11 313(7700)/2155(1595) measured(unique) transitions, belonging to 165/113/50 vibrational bands, respectively. The measured transitions collected from the literature span the regions 953–12 570 cm⁻¹ (for 828), 628–8197 cm⁻¹ (for 728), and 600–7918 cm⁻¹ (for 838). The number of critically evaluated and recommended energy levels of this study are 3923, 4318, and 1058 for the 828, 728, and 838 isotopologues of CO₂, respectively. Comparison of the empirical rovibrational energy levels determined in this study with their counterparts in two published databases, CDSD-2019, Ames-2021 and HITRAN shows very good overall agreement.

The current study involves an ab initio exploration of the ground and low-lying excited electronic states of the rhodium halide molecules RhF and RhCl using the complete active space self-consistent field (CASSCF) with multireference configuration interaction (MRCI+Q) method including single and double excitations and with Davidson corrections. We investigated the potential energy curves, the transition and permanent electric dipole moments, the electronic energy relative to the ground state T_e, the harmonic frequency ω_e, the internuclear distance R_e, and the rotational constant B_e corresponding to each of the bounded states. Our findings demonstrate good agreement with the available experimental data. Notably, this work represents the inaugural theoretical investigation of the excited states of RhF and RhCl molecules, identifying the ground state of both to be X³Π, as observed in the sole two experimental investigations.

Torsional vibrational states of trans − and cis − conformers belonging to point symmetry groups C_2H and C_2V, respectively, of hydroquinone molecules are classified according to the irreducible representations of the molecular symmetry group D_2H(M). A correspondence has been established between the symmetry elements of the D_2H(M) group and the symmetry elements on the torsional coordinate plane for two-dimensional surfaces of potential energy, wave functions, kinetic coefficients and dipole moment projections. A correspondence has been established between the symmetry species of the point symmetry groups C_2H, C_2V and the symmetry species of the molecular symmetry group D_2H(M). Conformational states, barriers to internal rotation and the above-mentioned characteristics of the hydroquinone molecule were calculated at the MP2/Aug-cc-pVDZ, MP2/Aug-cc-pVQZ, MP2/Aug-cc-pVTZ, MP2/CBS(aD,aT,aQ), and CCSD(T)/dAug-cc-pVDZ levels of theory. The calculated data sets were approximated using symmetry-adapted sets of basis functions. Using a numerical solution of the vibrational Schrödinger equation of restricted dimensionality, the energies and wave functions of 50 stationary torsional states of the hydroquinone molecule were determined for the first time. The values of tunneling splittings of the ground vibrational and a number of excited torsional states of trans − and cis − conformers were determined. In particular, when calculating at the MP2/CBS(aD,aT,aQ) level of theory, the values of tunneling splittings of the ground vibrational states of trans − and cis − conformers turned were 1.32*10^-6 and 1.62*10^-6 cm⁻¹, which is consistent with the experimentally established upper limit for this value in the cis − conformer by authors of [W. Caminati, S. Melandri, L. B. Favero, J.Chem.Phys., 100 (1994) 8569 – 8572] (0.2 MHz or 6.67*10^-6 cm⁻¹). The calculations of the matrix elements of the dipole moment operator and the partition function made it possible to simulate the torsional IR spectra of the molecule’s conformers at different temperatures. The frequencies of fundamental torsional vibrations in the trans – (267.1 and 269.0 cm⁻¹) and cis – (269.5 and 270.9 cm⁻¹) conformers, calculated at the MP2/CBS(aD,aT,aQ) level of theory, are in good agreement with the experimental value of frequency of this vibration (266 cm⁻¹), established in [W.G. Fateley, G.L. Carlson, F.F. Bentley, J.Phys.Chem., 79 (1975) 199–204.].

A new program, westerfit, has been developed to treat $C_s$ molecules with internal rotation and spin angular momentum. It implements a single diagonalization Rho Axis Method approach for the torsion–rotation alongside a complete treatment of nuclear quadrupole interaction and spin–rotation coupling. Unlike other programs designed for internal rotation with spin effects, westerfit includes matrix elements off-diagonal in the rotational angular momentum quantum number, $N$, rather than the perturbative treatment of the spin–rotation and quadrupole interactions. This full combined approach allows fitting of all symmetrically allowed terms in both the spin–rotation and the quadrupole tensors as well as inclusion of higher order terms coupling the large amplitude motion to the spin angular momentum. The program was benchmarked against other published programs to test molecular cases of torsion–rotation, spin–rotation, and spin–torsion-rotation. All three tests produced a lower RMS. westerfit paves a way forward for complete treatment of spin–torsion–rotation problems regardless of barrier height or quadrupole moment.

Caffeine, theophylline and theobromine are representative xanthine alkaloids, commonly used as stimulants due to their effects on the central nervous system. Despite their similar molecular structures, they have different pharmacological effects, necessitating a rapid and accurate identification method. In this study, terahertz time-domain spectroscopy (THz-TDS) was used to measure the absorption spectra of these three xanthine alkaloids within the range of 2.0–17.0 THz. The characteristic absorption peaks were visualized and analyzed basing on the quantum chemical calculations using Hartree-Fock (HF), Møller–Plesset perturbation theory (MP2) and density functional theory (DFT). Caffeine exhibited unique absorption peaks at 4.24, 5.00, and 11.13 THz. Theophylline showed characteristic peaks at 9.25, 12.20, and 15.09 THz. While theobromine exhibited characteristic peaks at 4.45, 7.68, and 11.21 THz. The results demonstrate that combining THz-TDS with DFT calculation can non-destructively, efficiently, and accurately identify these xanthine alkaloids, and providing valuable information for further understanding their pharmacological functions.

Chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy has been used to measure the spectra of ten previously unobserved fluoroethylene (FE)/CO₂ clusters, (FE)_x(CO₂)_y, with x from 1 to 4 and y from 0 to 4. Multiple spectra were recorded with varying concentrations of CO₂ in the sample, and at least 400,000 free induction decays averaged per data set. This allowed implementation of data-centered approaches, using intensity variation, to identify subsets of transitions belonging to the same cluster species or those of similar composition, simplifying the assignment process for the complex mixture of clusters present. All spectra were fitted to Watson A-reduction Hamiltonians, including quartic distortion constants, with very few species requiring higher order distortion constants for satisfactory fits to be obtained. Computational data at MP2/6-311++G(2d,2p) and ωB97X-D/6-31+G(d,p) levels suggested FE:CO₂ ratios and likely structural arrangements of each cluster based on comparisons with experimental rotational constants.

The ground state rotational spectrum of 1,1-dichloro-1-silacyclohex-2-ene has been recorded using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. Several isotopologues in their natural abundances have been observed in the free-jet expansion, and their spectra assigned, making it possible to present a partial heavy-atom substitution structure. Furthermore, the high resolution of this technique allows the complicated hyperfine splitting pattern to be largely deconvoluted. As a result, the on-diagonal nuclear quadrupole coupling constants for the two chlorine atoms have been determined for all observed isotopologues. Additionally, χ_bc is determined for both chlorine atoms of the parent species. The quadrupole coupling tensors for the parent species have been diagonalised, noting some assumptions have been made pertaining to the off-diagonal nuclear quadrupole coupling constants in the principal axis system, to yield reasonable values of χ_zz and η which are then compared.

Microwave spectra of acetic anhydride, D6-acetic anhydride, and acetic difluoroacetic anhydride have been observed in a supersonic jet. In conjunction with accompanying DFT and MP2 calculations, these systems are shown to adopt a nonplanar configuration in which the C=O groups point in approximately orthogonal directions. Methyl group internal rotation was fully analyzed for both species. The observed conformation of these systems appears to result from an interaction between a CH₃ hydrogen (in acetic anhydride) or the CF₂H hydrogen (in acetic difluoroacetic anhydride) with the carbonyl group to which it is not directly bound, forming a six-membered ring. The fitted rotational constants for both systems are in reasonably good agreement with calculated values, but for acetic anhydride, the agreement is somewhat worse than that previously observed for a series of syn anhydrides. The calculations indicate a pronounced flexing of the heavy atom frame as the CH₃ group in the six-membered ring undergoes internal rotation, and this likely influences the level of agreement between the theoretical and vibrationally averaged experimental constants. The other CH₃ group does not interact with a carbonyl oxygen because of its orientation in the molecule, and its internal rotation does not induce similar changes in the molecular frame. In the acetic difluoroacetic anhydride, it is the CF₂H hydrogen that interacts with its remote carbonyl oxygen, leaving the internally rotating CH₃ group unaffected by participation in a six-membered ring and giving rise to much smaller deviations in the rotational constants as it moves along its internal rotation coordinate. Correspondingly better agreement between experimental and theoretical spectroscopic constants is obtained.